As optical transmission distances become longer and photonic networks begin to assume some of the routing functions formerly associated with electronic switching, there is a need for optical performance monitoring, and optical signal-to-noise ratio (OSNR) measurement in particular.
Direct spectrum measurement using tunable filters cannot distinguish between coherent signal power and incoherent noise power. It must rely on baseline measurements at signal-free wavelengths, but such baseline wavelengths may not be present in advanced networks with optical add/drop functions.
Polarization-based techniques offer a way to reject a signal so that the noise can be measured, but are susceptible to errors induced by partial polarization of the noise and by depolarization of the signal.
There remains a need for a system that is well suited to signal integrity monitoring and diagnostics, and that permits measurement of OSNR.